TiAl-based intermetallic compound with excellent high temperature strength

ABSTRACT

A TiAl-based intermetallic compound has a metallographic structure which includes a region A having fine β-phases dispersed in a γ-phase. The volume fraction Vf of the β-phases in the region A is set equal to or more than 0.1% (Vf≧0.1%). Thus, the β-phases can exhibit a pinning effect to prevent a transgranular pseudo cleavage fracture in the γ-phase, thereby providing an enhanced high-temperature strength of the TiAl-based intermetallic compound.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a TiAl-based intermetallic compoundhaving an excellent high-temperature strength, and processes forproducing the same.

2. Description of the Prior Art

A TiAl-based intermetallic compound is expected as a lightweight heatresistant material, and those having various structures have beenconventionally proposed (for example, see U.S. Pat. No. 4,879,092 andJapanese Patent Application Laid-open Nos. 25534/90 and 193852/91).

However, even now conventional TiAl-based intermetallic compounds arenot put into practical use as a heat-resistant material, because thestrength thereof is insufficient for high temperatures. That istemperatures exceeding about 750° C.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aTiAl-based intermetallic compound of the type described above, which hasa high-temperature strength enhanced by improving the metallographicstructure thereof, and a process for producing the same.

To achieve the above object, according to the present invention, thereis provided a TiAl-based intermetallic compound with an excellenthigh-temperature strength, wherein the compound has a metallographicstructure which comprises a region having fine β-phases dispersed in aγ-phase, the volume fraction Vf of the β-phases in the region beingequal to or more than 0.1% (Vf≧0.1%).

If the metallographic structure of the TiAl-based intermetallic compoundis configured in the above manner, it is possible to enhance thehigh-temperature strength of the TiAl-based intermetallic compound. Thisis attributable to the fact that the fine β-phases dispersed in theγ-phase exhibit a pinning effect, thereby preventing a transgranularpseudo cleavage fracture in the γ-phase. However, if the volume fractionVf of the β-phases is less than 0.1%, a sufficient pinning effect cannotbe provided. If the β-phases are present between the adjacent regions,i.e., in the grain boundaries, a high-temperature strength enhancingeffect is not provided.

In addition, according to the present invention, there is provided aprocess for producing a TiAl-based intermetallic compound with anexcellent high-temperature strength, having a metallographic structurewhich comprises; a first region consisting of either a region havingfine β-phases dispersed in a γ-phase, or a region consisting of α₂-phases and fine β-phases dispersed in a γ-phase; and a second regionhaving a γ-phase which does not include β-phase, the volume fraction Vfof β-phases in the first region being equal to or more than 0.1%(Vf≧0.1%); the process comprising: a first step of subjecting aTiAl-based intermetallic compound blank having a metallographicstructure including a γ-phase and at least one of α₂ - and β-phases to asolution treatment at a treatment temperature set in a range whichpermits α- and γ-phases to be present, thereby providing an intermediateproduct having a metallographic structure including γ-phases andsupersaturated α₂ -phases, and a second step of subjecting theintermediate product to an artificial aging treatment at a temperatureset in a range which permits α₂ - and γ-phases to be present.

In the above producing process, if the TiAl-based intermetallic compoundblank is subjected to the solution treatment employing the treatmenttemperature and a quenching, it is possible to prevent a coalescence ofα₂ - and γ-phases in the intermediate product. If the intermediateproduct is subjected to the artificial aging treatment at theabove-described temperature, the γ-phase is precipitated in the α₂-phase, and the fine β-phases are precipitated in a dispersed fashion inthe γ-phase. Further, depending upon the treatment temperature in thesolution treatment, the α₂ -phases may be dispersed together with theβ-phases in the γ-phase.

The above and other objects, features and advantages of the inventionwill become apparent from the following description of preferredembodiments, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating one example of ametallographic structure of a TiAl-based intermetallic compound;

FIG. 2 is a schematic diagram illustrating another example of ametallographic structure of a TiAl-based intermetallic compound;

FIG. 3 is a portion of a phase diagram showing states of the TiAl-basedintermetallic compound;

FIG. 4A is a photomicrograph showing a metallographic structure of aTiAl-based intermetallic compound according to an example of the presentinvention;

FIG. 4B is a schematic tracing of an essential portion shown in FIG. 4A;

FIG. 5A is a photomicrograph showing a metallographic structure of aTiAl-based intermetallic compound according to a comparative example;

FIG. 5B is a schematic tracing of an essential portion shown in FIG. 5A;

FIG. 6 is a photomicrograph showing a metallographic structure of aTiAl-based intermetallic compound blank;

FIG. 7 is a graph illustrating the relationship between the temperatureand the 0.2% yield strength; and

FIG. 8 is a graph illustrating the relationship between the volumefraction Vf of a first region and the elongation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, one example of a metallographic structure of aTiAl-based intermetallic compound is illustrated in a schematic diagram.This metallographic structure is comprised of an infinite number ofregions A each having fine β-phases (β-phases having B2 orderedstructure) dispersed in a γ-phase (a TiAl phase). In addition to theβ-phases, α₂ -Phases may be dispersed in the γ-phase in some cases.

With such a configuration, the fine β-phases dispersed in the γ-phaseexhibit a pinning effect, and a transgranular pseudo cleavage fracturein the γ-phase is prevented, thereby enhancing a high-temperaturestrength of a TiAl-based intermetallic compound. The volume fraction Vfof the β-phases in each of the regions A is set equal to or more than0.1% (Vf≧0.1%) in order to provide such effect. It should be noted thatthe α₂ -phases dispersed in the γ-phase do not contribute to anenhancement in high-temperature strength of the TiAl-based intermetalliccompound.

FIG. 2 is a schematic diagram showing another example of ametallographic structure of a TiAl-based intermetallic compound. Thismetallographic structure is comprised of an infinite number of firstregions A each having fine β-phases dispersed in a γ-phase, and aninfinite number of regions B each having a γ-phase with no β-phaseincluded therein. In the first region A, α₂ -phases, in addition to theβ-phases, may also be dispersed in the γ-phase in some cases.

Even with such a configuration, an effect similar to the above-describedeffect is provided because of the presence of the first regions A. Inorder to provide such effect, the volume fraction Vf of the β-phases ineach of the regions A is set equal to or more than 0.1% (Vf≧0.1%), andthe volume fraction Vf of the first regions A in the metallographicstructure is set equal to or more than 1% (Vf≧1%). It should be notedthat the γ-phase including no α₂ - and β-phases and thus, the secondregion B does not contribute to the enhancement in high-temperaturestrength of the metallographic structure.

A difference between the metallographic structures of theabove-described types is attributable to conditions for producing theTiAl-based intermetallic compounds. For example, in producing theTiAl-based intermetallic compound having the metallographic structureshown in FIG. 2, there is employed a procedure, which comprises a firststep of subjecting a TiAl-based intermetallic compound blank having ametallographic structure including a γ-phase and at least one of α₂ -and β-phases to a solution treatment at a treatment temperature which isset in a range permitting the α₂ and γ-phases to be present, therebyproviding an intermediate product having a metallographic structureincluding the γ-phase and supersaturated α₂ -phases; and a second stepof subjecting the intermediate product to an artificial aging treatmentat a treatment temperature which is set in a range permitting the α₂ -and γ-phases to be present. The TiAl-based intermetallic compound blankcontains aluminum in a content represented by 36 atomic % ≦Al≦52 atomic% and titanium in a content represented by 48 atomic % ≦Ti≦64 atomic %as well as at least one β-area enlarging element E as a third element,which is selected from the group consisting of Mo, Nb, Ta, V, Co, Cr,Cu, Fe, Mn, Ni, Pb, Si and W. The content of the β-area enlargingelement E is set equal to or more than 0.5 atomic %. If the contents ofaluminum, titanium and the β-area enlarging element E depart from theabove-described ranges, respectively, it is not possible to produce aTiAl-based intermetallic compound blank having a metallographicstructure of the type described above.

As shown in FIG. 3 the treatment temperature in the solution treatmentis set at a range equal to or more than an eutectoid line E_(L) whichpermits a reaction, α-phase+γ-phase→α₂ -phase+γ-phase, to occur, but isset equal to or less than α-transus line T_(L) which permits a reaction,α-phase→α-phase+γ-phase, to occur, in a Ti-Al based phase diagram. Thisis for the purpose of preventing the coalescence of the α₂ - andγ-phases in the intermediate product.

The cooling rate in the solution treatment is set at a value higher thana cooling rate in an oil quenching. This is because γ-phases may beprecipitated in a laminar configuration in an α₂ -phase, if the coolingrate is slower than that during an oil quenching.

The treatment temperature in the artificial aging treatment is set in arange equal to or more than 700° C., but equal to or less than theabove-described eutectoid line E_(l). In this range of temperature, fineβ-phases can be precipitated in a dispersed state in the γ-phase.

The heating time in the solution treatment and the artificial agingtreatment is set in a range of at least 5 minutes to ensure that thesetreatments are practically effective.

Particular examples will be described below.

First, a starting material was prepared by weighing an aluminum shothaving a purity of 99.99%, a titanium sponge having a purity of 99.8%and a Cr-Nb alloy, so that Al was 47 atomic %; Cr was 2 atomic %; Nb was2 atomic %, and the balance was titanium.

The starting material was melted in a plasma melting furnace to prepareabout 20 kg of an ingot. Then, the ingot was subjected to a homogenizingtreatment at 1200° C. for 48 hours for the purpose of homogenizing theingot and removing casting defects. Subsequently, the ingot wassubjected to a hot isostatic pressing treatment under conditions of1200° C., 3 hours and 193 MPa. Further, the resulting material wassubjected to an upsetting treatment with an upsetting rate of 80% (ahigh rate) at 1200° C. by a vacuum isothermal forging. The upset productobtained in this manner was cut into a plurality of TiAl-basedintermetallic compound blanks. The metallographic structure of theseTiAl-based intermetallic compound blanks was comprised of an infinitenumber of γ-phases, and β- and α₂ -phases precipitated in a grainboundary of the γ-phases. Each of the TiAl-based intermetallic compoundblanks was heated for 2 hours at 1200°-1300° C. and was then subjectedto a solution treatment in which a water-hardening was conducted,thereby providing an intermediate product. Each of the intermediateproducts has a metallographic structure having β-phases andsupersaturated α₂ -phases. No β-phase was precipitated in the γ-phase.

Then, individual intermediate products were subjected to an artificialaging treatment in which they were heated for 1 to 12 hours at900°-1200° C., thereby providing TiAl-based intermetallic compoundsaccording to examples of the present invention and comparative examples.

Table 1 shows conditions in the solution treatment and conditions in theartificial aging treatment for the examples (1) to (3) and thecomparative examples (1) and (2). The comparative example (2) isTiAl-based intermetallic compound blank.

                  TABLE 1                                                         ______________________________________                                                             Artificial Aging                                                Solution Treatment                                                                          Treatment                                                       Temperature                                                                            Time     Temperature                                                                              time                                             (°C.)                                                                           (hour)   (°C.)                                                                             (hour)                                    ______________________________________                                        Example (1)                                                                            1300       2        900      12                                      Example (2)                                                                            1200       2        900      8                                       Example (3)                                                                            1300       2        900      1                                       Comparative                                                                            1300       2        1200     3                                       example (1)                                                                   Comparative                                                                            --         --       --       --                                      example (2)                                                                   ______________________________________                                    

FIG. 3 shows a diagram showing states of the TiAl-based intermetalliccompound in the example (1) or the like and thus the TiAl-basedintermetallic compound having Cr and Nb contents set at 2 atomic %. Inthe examples (1) to (3), the treatment temperature in the solutiontreatment is set in a range equal to or more than the eutectoid lineE_(L), but equal to or less than the α-transus line T_(L). And thetreatment temperature in the artificial aging treatment is set in arange equal to or more than 700° C., but equal to or less than theeutectoid line E_(L). In the case of the comparative example (1), thetreatment temperature in the solution treatment is set in theabove-described range, but the treatment temperature in the artificialaging treatment exceeds the eutectoid line E_(L) which is the upperlimit value of the above-described range.

Table 2 shows textures on the metallographic structure for the examples(1) to (3) and the comparative examples (1) and (2)

                  TABLE 2                                                         ______________________________________                                               Vf of Vf of phases  Vf of phases                                              first dispersed in first                                                                          dispersed in grain                                        region                                                                              regions A (%) boundary (%)                                              A (%) β-phase                                                                           α.sub.2 -phase                                                                   β-phase                                                                         α.sub.2 -phase                    ______________________________________                                        Example (1)                                                                            82      5        0      0      0                                     Example (2)                                                                            75      2        1      0      0                                     Example (3)                                                                            60      0.5      0      0      0                                     Comparative                                                                             0      0        0      3      7                                     example (1)                                                                   Comparative                                                                             0      0        0      2      5                                     example (2)                                                                   ______________________________________                                    

FIG. 4A is a photomicrograph (2,000 magnifications) showing themetallographic structure of the example (1), and FIG. 4B is a schematictracing of an essential portion shown in FIG. 4A. This metallographicstructure corresponds to that shown in FIG. 2 and hence, has firstregions A each having γ- and β-phases, and second regions B each havinga γ-phase with no β-phase included therein.

FIG. 5A is a photomicrograph (2,000 magnifications) showing themetallographic structure of the comparative example (1), and FIG. 5B isa schematic tracing of an essential portion shown in FIG. 5A. In thismetallographic structure, α₂ - and β-phases are precipitated at thegrain boundary of each γ-phase, but no α₂ - and β-phases exist in theγ-phase.

FIG. 6 is a photomicrograph (500 magnifications) showing themetallographic structure of the comparative example (2). In FIG. 6,relatively white and small island-like portions are β-phases, more darkcolored and smaller island-like portions are α₂ -phases, and the otherportions are γ-phases. The β-phases and α₂ -phases are precipitated atthe grain boundary of the γ-phases, but no α₂ - and β-phases exist inthe γ-phase.

FIG. 7 shows results of a tensile test in a range of from ambienttemperature to 900° C. for the examples (1) to (3) and the comparativeexamples (1) and (2). In FIG. 7, a line a₁ corresponds to the example(1); a line a₂ to the example (2); a line a₃ to the example (3); a lineb₁ to the comparative example (1), and a line b₂ to the comparativeexample (2).

It can be seen from FIG. 7 that the examples (1) , (2) and (3) indicatedby the lines a₁, a₂ and a₃ have an excellent high-temperature strength,as compared with the comparative examples (1) and (2) indicated by thelines b₁ and b₂. In the examples (1), (2) and (3), the high-temperaturestrength is increased with an increase in volume fraction Vf of theβ-phases in the first region A. Especially in the case of the examples(1) and (2) indicated by the lines a₁ and a₂, the high-temperaturestrength is higher than the ambient-temperature strength at about 660°to about 880° C., and the maximum strength is shown at 800° C.

In the TiAl-based intermetallic compound of this type, the volumefraction Vf of β-phases is set equal to or more than 0.1% (Vf≧0.1%) inorder to insure a high-temperature strength attributable to the presenceof the β-phases.

Table 3 shows the conditions in the solution treatment, the volumefraction Vf of the first regions A, the volume fraction of the β-phasesin the first regions A, and the elongation for examples (4) to (8) and acomparative example (3). The artificial aging treatment was carried outat 900° C. for 12 hours.

                  TABLE 3                                                         ______________________________________                                                                    Vf of β-                                                         Vf of   phases                                                   Solution Treatment                                                                         first   in first Elon-                                           Temperature                                                                            Time    region  region gation                                        (°C.)                                                                           (hour)  A (%)   (%)    (%)                                    ______________________________________                                        Example (4)                                                                            1250       2       39    4.5    1.3                                  Example (5)                                                                            1280       2       31    4.0    1.2                                  Example (6)                                                                            1300       2       15    2.0    1.0                                  Example (7)                                                                            1320       2        5    1.8    0.8                                  Example (8)                                                                            1340       2        2    0.2    0.25                                 Comparative                                                                            1400       2        0    0      0.2                                  example (3)                                                                   ______________________________________                                    

FIG. 8 is a graph taken from the relationship shown in Table 3, whereinspots (4) to (8) and (3) correspond to the examples (4) to (8) and thecomparative example (3), respectively.

It is apparent from FIG. 8 that the elongation of the TiAl-basedintermetallic compound has a point of inflection at about 1% the volumefraction Vf of the first regions A. Therefore, in order to insure aductility of a TiAl intermetallic compound, the volume fraction of thefirst regions A is set equal to or more than 1% (Vf≧1%).

What is claimed is:
 1. A TiAl-based intermetallic compound with anexcellent high-temperature strength, wherein said compound has ametallographic structure which comprises a region having fine β-phasesdispersed in a γ-phase, the volume fraction Vf of β-phases in saidregion being equal to or more than 0.1% (Vf≧0.1%).
 2. A TiAl-basedintermetallic compound with an excellent high-temperature strengthaccording to claim 1, wherein α₂ -phases are dispersed in the γ-phase insaid region.
 3. A TiAl-based intermetallic compound with an excellenthigh-temperature strength according to claim 1, wherein the volumefraction (Vf) of said region in said metallographic structure is greaterthan or equal to 1%.
 4. A TiAl-based intermetallic compound with anexcellent high-temperature strength, wherein said compound has ametallographic structure which comprises a first region having fineβ-phases dispersed in a γ-phase, and a second region having a γ-phasewhich does not include β-phase, the volume fraction Vf of the β-phasesin said first region being equal to or more than 0.1% (Vf≧0.1%).
 5. ATiAl-based intermetallic compound with an excellent high-temperaturestrength according to claim 4, wherein α₂,phases are dispersed in theγ-phase in said first region.
 6. A TiAl-based intermetallic compoundwith an excellent high-temperature structure according to claim 4,wherein the volume fraction (Vf) of said first region in saidmetallographic structure is greater than or equal to 1%.
 7. A TiAl-basedintermetallic compound with an excellent high-temperature strengthaccording to claim 4 or 5, wherein the volume fraction Vf of said firstregion in said metallographic structure is equal to or more than 1%(Vf≧1%).
 8. A TiAl-based intermetallic compound with an excellenthigh-temperature strength according to claim 1, 2, 4 or 5, furtherincluding at least one β-area enlarging element E selected from thegroup consisting of Mo, Nb, Ta, V, Co, Cr, Cu, Fe, Mn, Ni, Pb, Si and W,the content of said β-area enlarging element E being equal to or morethan 0.5 atomic % (E≧0.5 atomic %).